Immune competence of the mammary gland as affected by somatic cell and pathogenic bacteria in ewes with subclinical mastitis

Abstract

Immune competence of the ewe mammary gland was investigated by monitoring the leukocyte differential count, cytokine pattern, and endogenous proteolytic enzymes in milk samples with different somatic cell counts (SCC) and pathogenic bacteria. Furthermore, the leukocyte differential count and T-lymphocyte populations were evaluated in ewe blood. A total of 1,500 individual milk samples were randomly selected from the pool of the samples collected during sampling and grouped into 5 classes of 300 samples each, on the basis of SCC. Classes were <300,000cells/mL, from 300,000 to 500,000cells/mL, from 501,000 to 1,000,000cells/mL, from 1,001,000 to 2,000,000cells/mL, and >2,000,000cells/mL. Microbiological analyses of ewe milk were conducted to detect mastitis-related pathogens. Sheep whose udders were without clinical abnormalities, and whose milk was apparently normal but with at least 103cfu/mL of the same pathogen were considered to have subclinical mastitis and therefore defined as infected. Polymorphonuclear neutrophilic leukocytes (PMNL) and macrophages increased with SCC, whereas lymphocytes decreased. Milk samples with SCC >1,000,000cells/mL showed differences in leukocyte populations between uninfected and infected ewes, with higher percentages of PMNL and macrophages and lower percentages of lymphocytes in infected animals. Nonviable PMNL levels were the highest in ewe milk samples with SCC <300,000cells/mL; starting from SCC >500,000cells/mL, nonviable PMNL were higher in uninfected ewes than in infected ones. In infected animals giving milk with SCC >1,000,000cells/mL, a higher CD4+/CD8+ ratio was observed, suggesting that the presence of pathogens induced an activation of both CD4+ and CD8+. The levels of tumor necrosis factor-α and IL-12 were higher in infected than uninfected ewes, irrespective of SCC. Plasmin activity increased along with SCC and was always higher in infected than uninfected animals; cathepsin D increased starting from 1,001,000cells/mL in milk samples from noninfected ewes and starting from 301,000cells/mL in milk samples from infected animals. The associations between somatic cells, cytokines, endogenous proteolytic enzymes, and pathogenic bacteria can be used to better understand the pathogenesis of subclinical mastitis in ewes and the effect on the immune response of ewe mammary gland.